Why California’s 'El Niño = Wet, La Niña = Dry' Rule Is Breaking Down

NOAA: Weak La Niña persists as winter approaches

NOAA data released Thursday shows cooler-than-average sea-surface temperatures across the central and eastern equatorial Pacific, with most models forecasting a weak La Niña to persist into winter. But that slow ocean signal no longer provides a reliable shorthand for California's winter weather.

ENSO sets a background, not a script

El Niño and La Niña are opposite phases of the El Niño–Southern Oscillation (ENSO), a climate cycle that nudges large-scale atmospheric patterns. Forecasters have long used ENSO as a seasonal indicator: warm equatorial Pacific waters favor El Niño, cool waters favor La Niña. As Michelle L'Heureux of NOAA's Climate Prediction Center put it, "ENSO sets the background — it shifts probabilities, it doesn't guarantee outcomes."

Why the simple rule fails

The connection between ENSO and California precipitation weakened after the 1990s. Rosa Luna Niño of Scripps notes the correlation between Pacific conditions and California precipitation was strongest from about 1970 to 1990 and has since declined. Faster, subseasonal phenomena — especially the Madden–Julian Oscillation (MJO) and atmospheric rivers — often dominate, steering storms on timescales of weeks rather than months.

Recent examples

2022–23: A rare "triple-dip" La Niña typically suggests a dry winter, yet eleven atmospheric rivers slammed California in late December and January, delivering large volumes of rain and snow over a few weeks. A strong MJO amplified the jet stream and tapped subtropical moisture, overpowering the La Niña background.

2015–16: One of the strongest El Niño events on record did not translate into an exceptionally wet California winter because a persistent high-pressure block steered storms north. Later research implicated unusually warm Indian Ocean waters and related convective activity in nudging the jet stream.

Measurement and baseline issues

Another complication is a shifting baseline: as the Pacific warms, the 30-year "normal" sea-surface temperatures used to classify ENSO events have risen, blurring thresholds and sometimes misclassifying marginal events. NOAA and the Australian Bureau of Meteorology are revisiting how ENSO is calculated and categorized to account for that drift.

What forecasters are doing now

Rather than relying on ENSO alone, researchers and operational centers now combine the slow ENSO background with subseasonal predictors and wind/jet-stream patterns. At Scripps's Center for Western Weather and Water Extremes (CW3E), scientists layer ENSO with the MJO and atmospheric-river probabilities; their Atmospheric River Scale helps translate complexity into actionable guidance for flood forecasters and reservoir managers. NCAR groups wet winters by recurring jet-stream configurations that can sometimes be predicted weeks ahead, bridging ENSO's seasonal signal and the weather people experience.

Takeaway

ENSO remains a useful seasonal indicator, but it is not a deterministic rule for California's winter weather. Week-to-week patterns — the winds and jets that actually steer storms — often decide whether a season is wet or dry. Forecasters now emphasize pattern-based, coupled approaches that link the slow oceanic background with faster atmospheric dynamics to give more locally relevant guidance.

"ENSO sets the background, but for this region the week-to-week patterns decide the impacts," — Michelle L'Heureux, NOAA.